<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{margin: 0; padding: 0;}
        .box{
            width: 500px;
            height: 500px;
            background-color: red;
            margin: 0 auto;
            /* position: relative; */
            position: absolute;
            top: 50%;
            left: 50%;
            margin-left: -250px;
            margin-top: -250px;
        }
        .child{
            width: 250px;
            height: 250px;
            background-color: green;
            position: absolute;
            top: 50%;
            left: 50%;
            margin-left: -125px;
            margin-top: -125px;        
        }
    </style>
</head>
<body>
    <div class="box">
        <div class="child"></div>
    </div>
</body>
</html>